Efficient mid-infrared (mid-IR) light sources and lasers emitting at wavelengths between about 3 and about 5 microns would be desirable enablers for a variety of technologies, including gas analysis, remote sensing, and atmospheric monitoring. However, among current sources, organic laser dyes are poor fluorophores (quantum yields in emission of less than 0.5%) that are confined to the near-infrared (near-IR), while mid-IR emitting rare-earth doped crystals offer limited emission wavelengths and small absorption cross sections. Alternatively, epitaxial quantum wells provide the possibility for efficient, continuously tunable mid-IR emission, but are not chemically processible, according to Hollingsworth, J., and V. Klimov, “Pushing the Band Gap Envelope: Mid-Infrared Emitting Colloidal PbSe Quantum Dots,” J. Am. Chem. Soc., 126, 11752-11753 (2004).
Atmospheric absorption of light primarily due to oxygen, carbon dioxide, and water vapor results in the atmospheric transmission spectra depicted in FIG. 1. As can be identified on the spectra, there exist specific wavelength windows where light can travel long distances without appreciable losses. Aside from visible light, infrared light having wavelengths between about 3 and about 5 microns is generally absorbed little or not at all by the atmosphere. Light of this wavelength range can travel long distances through the atmosphere without being substantially absorbed, thereby making this range attractive in providing efficient mid-IR light sources and lasers.
The relationship between the peak emission wavelength and the temperature of a black body is defined by Wien's displacement law (Equation 1), which states:
                                          λ            max                    =                      b            T                          ,                            (        1        )            where λmax is the peak wavelength of the black body in meters (m), T is the temperature of the black body in degrees Kelvin (K), and b is a constant of proportionality called Wien's displacement constant and equals 2.8977685(51)×1-3 m·K. The parenthesized digits in b denote the uncertainty in the two least significant digits, where uncertainty is measured as the standard deviation at 68.27% confidence level.
Wien's displacement constant b equals 2.8977685(51)×106 nm·K, when defined in units of nanometers (nm) rather than meters (m), as is typically done for optical wavelengths.
Basically, Wien's displacement law shows that the higher the object's temperature, the shorter the wavelength at which the object's peak emissions occur. For example, the sun's surface temperature is 5778 K and the sun's peak emission wavelength is 502 nm, which is about midway the visual light spectrum. This is why the visible light emitted by the sun is basically white light. As the light hits the Earth's atmosphere, it is subject to Rayleigh scattering, which results in an observed blue sky and yellow sun.
Wien's displacement law in the frequency domain (Equation 2) states:
                                          f            max                    =                                                                      α                  ⁢                                                                          ⁢                  k                                h                            ⁢              T                        ≈                                          (                                  5.879                  ×                                      10                    10                                    ⁢                                                                          ⁢                  Hz                  ⁢                                      /                                    ⁢                  K                                )                            ·              T                                      ,                            (        2        )            where fmax is the peak emission frequency of the black body in hertz (Hz), a 2.821439 . . . , a constant resulting from the numerical solution of the maximization equation, k is Boltzmann's constant, h is Planck's constant, and T is the temperature of the black body in degrees Kelvin (K).
In terms of Boltzmann's constant k, and Planck's constant h, Wien's displacement law (Equation 3) becomes:
                                          λ            max                    =                                                    hc                kx                            ⁢                              1                T                                      =                                          2.89776829                ⁢                                                                  ⁢                …                ×                                  10                  6                                ⁢                                                                  ⁢                                  nm                  ·                  K                                            T                                      ,                            (        3        )            where λmax is the peak emission wavelength of the black body in nanometers (nm), h is Planck's constant, c≈3.0×108 m/s, the speed of light, k is Boltzmann's constant, x≈4.96511 . . . , a dimensionless constant, and T is the temperature of the black body in degrees Kelvin (K).
Hence, Wien's displacement law may be used to determine at what temperature a light source may emit light at wavelengths within the 3 to 5 micron range or vice versa.
Research conducted at Los Alamos by Hollingsworth and Klimov has shown that semiconductor nanocrystals of the appropriate size and composed of a sufficiently small bandgap semiconductor lead selenide (PbSe) can emit, e.g., exhibit photoluminescence, in the mid-IR wavelengths (they demonstrated peak emission at wavelengths as long as 4 microns) when illuminated with a shorter wavelength source. However, the quantum yield (i.e., the percent of absorbed photons that are reemitted as photons) was very poor (less than 1% compared to greater than 50% for shorter wavelength emitters) due to strong mid-IR absorption of the organic surfactants that surround each semiconductor nanocrystal. Further, water, carbon dioxide and hydroxide groups found in organic surfactants have some absorption in mid as well as near infrared wavelengths.
Semiconductor nanocrystals are typically crystalline particles of II-VI, III-V, or II-VI semiconductor material consisting of hundreds to thousands of atoms. They are neither atomic nor bulk semiconductors, but may best be described as artificial atoms. Their properties originate from their physical size, which ranges from approximately 10 to 100 Å (10−10 meters) in radius and is often comparable to or smaller than the bulk exciton Bohr radius. As a consequence, semiconductor nanocrystals no longer exhibit their bulk parent optical or electronic properties. Instead, they exhibit electronic properties due to what are commonly referred to as quantum confinement effects. These effects originate from the spatial confinement of intrinsic carriers (electrons and holes) to the physical dimensions of the material rather than to bulk length scales. One of the better-known confinement effects is the increase in semiconductor bandgap energy with decreasing particle size; this manifests itself as a size dependent blue shift of the band edge absorption and luminescence emission with decreasing particle size.
As nanocrystals increase in size past the exciton Bohr radius, they become electronically and optically bulk-like. Therefore nanocrystals should not be made to have a smaller bandgap than exhibited by the bulk materials of the same composition, which means that the longest wavelength that can be emitted by a nanocrystal is equivalent to the bulk bandgap energy.
Each individual nanocrystal emits a light with a line width comparable to that of atomic transitions. Any macroscopic collection of nanocrystals, however, emits a line that is inhomogeneously broadened due to the fact that every collection of nanocrystals is unavoidably characterized by a distribution of sizes. For example, in a collection of CdSe, InGaP, and PbS nanocrystals with size distributions exhibiting roughly a minimum 5% variation in nanocrystal volume, the width of the inhomogeneously broadened line corresponds to ˜35 nm for CdSe, ˜70 nm for InGaP, and ˜100 nm for PbS.
The absorption spectra of nanocrystals are dominated by a series of overlapping peaks with increasing absorption at shorter wavelengths. Each peak corresponds to excitonic energy level, where the first exciton peak (i.e. the lowest energy state) is synonymous with the blue shifted band edge. Short wavelength light that is absorbed by the nanocrystals will be down converted and reemitted at a shorter wavelength. The efficiency at which this down conversion process occurs is denoted by the quantum yield. Nonradiative exciton recombination reduces quantum yield and is due to the presence of interband states resulting from dangling bonds at the nanocrystal surface and intrinsic defects.
Accordingly, there is a need in the art to develop a stable semiconductor nanocrystal that is brightly fluorescing at shorter peak emission wavelengths.